1. Field of the Invention
This invention relates to semiconductor varistors and a method of making the same.
2. Description of the Prior Art
Varistors are nonlinear resistive elements commonly used to protect electrical circuits or electrical apparatus from high voltage surges. In particular, it is often desirable to protect power lines from high voltage surges to protect the transformers, insulators and electrical equipment associated therewith. One type of prior art varistor embodies a polycrystalline material such, for example, as a zinc oxide-bismuth oxide combination. However, it has been discovered that materials such as this combination experience an increase in leakage current with the passage of time. One could use a plurality of several connected back-to-back Zener diodes but the economics of such a system is prohibited. Additionally, each electrical connection is a potential source of failure particularly if a sound electrical connection has not resulted from the joining operation.
An object of this invention is to provide a new and improved semiconductor varistor which corrects the deficiencies of the prior art devices.
Another object of this invention is to provide a new and improved semiconductor varistor having a lamellar structure including spaced regions of recrystallized substrate material.
Another object of this invention is to provide a new and improved semiconductor varistor having a lamellar structure including spaced regions of recrystallized substrate material having a substantially uniform dopant impurity concentration through the regions.
Other objects of this invention will, in part, be obvious and will, in part, appear hereinafter.
In accordance with the teachings of this invention there is provided a semiconductor varistor comprising a body of semiconductor material having a lamellar structure of a plurality of integral diodes arranged in a series circuit. Each diode has first and second regions of different and opposite type conductivity. The first region has a first type conductivity and a selected resistivity. The second region has recrystallized material of the first region including a sufficient quantity of a dopant material to impart a second and the opposite conductivity type thereto. A first P-N junction is formed by the contiguous surfaces of each pair of first and second regions of opposite type conductivity of each diode. A second P-N junction is formed by the contiguous surfaces of each pair of regions of opposite type conductivity of each pair of mutually adjacent integral diodes which are in abutting contact with each other. The varistor has an inherent physical characteristic to break down and conduct an electrical current upon the application of a selected voltage across the P-N junction of the lamellar body.